Abstract: A transit time mass spectrometer for analyzing ions having different energies, particularly ions emitted under the influence of radiation, especially laser radiation. The mass spectrometer is provided with a device forming a drift path having input and output ends and adapted to be traversed by the ions in a predetermined direction, with an ion detector which is located downstream of the output end of the drift path, and with a lens arrangement, as, for example, as electrostatic or magnetic lens system for focussing as well as for accelerating the ions. The lens system has at least two sections at least one of which is upstream of the input end.